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cabinbiscuits
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# Posted: 31 Jan 2012 09:31pm
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I'm starting to plan my foundation and floor framing and would like to run my ideas past you folks on here that know a lot more about building than I do.
I'm building a 16 x 20 building which will sit on a pier and girder type of foundation. The building will have a gambrel roof with either a full or half loft ( haven't decided for sure on that yet) which will contain beds, and minimal furniture.
My current plan is to use foundation grade pressure treated 6x6 posts (I can't find 8x8's anywhere around my area for some reason) along the 20' sides of the building. I figured on boring 18 inch holes about 48" deep and setting each post on top of a rebar reinforced concrete pad at the bottom, and then backfill and tamp with dirt. I plan on spacing the posts at 5' which would mean there will be five posts supporting each 20' side of the building. The site where the building will be is relatively flat, and I plan on having the top of the girder be about 24" above grade at the highest point of the grade under the building.
For the girders I am thinking of using 20' 6x6's on top of the piers. At this point I am thinking that I will not use a middle girder under the building, and will instead use 16' 2x12's to span the entire width of the building.
As stated above, I plan on using pressure treated 2x12x16's spaced on 16" centers for the floor to span the entire floor, with pressure treated plywood nailed to the bottom of the floor framing to close in the bottom of the floor. The top of the floor will be either 3/4" OSB or plywood.
The wall framing will be 2x4's on 16" centers.
So here's some of my questions--
Should I use something other than the 6x6's for the girders?
Does this plan seem like overkill or even worse inadequate?
Using the link for the joist calculator from this site, using 2x10's on 16's or 2x12's at a 24" spacing would almost work, but I'm not really crazy about having the building cantilevered out over the girders. The factors I used for the calculator were L/360 40PSF 10 PSI Dead Load.
Am I wasting money doing it the way I am thinking instead of downsizing the floor joists and adding a third girder in the center of the building, or just cantilevering out over the girders?
Thanks in advance for your help, as well as all of the information that I've gleaned from other posts to get to this point.
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MtnDon
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# Posted: 31 Jan 2012 10:25pm
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any idea what you snow load is in lb/sq ft (ground) ??
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cabinbiscuits
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# Posted: 31 Jan 2012 10:36pm
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From what I can tell on line it looks like Pennsylvania is 25-35 PSF, so I would figure the worst case scenario at 35 PSF.
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MtnDon
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# Posted: 31 Jan 2012 11:06pm - Edited by: MtnDon
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Assuming a snow load of up to 30 PSF... and being conservative and counting the loft floor as a full clear span floor ...
Two 2x8 can span up to 5'0" as a girder. Two 2x10 can span up to 6'1". that's using #2 grade hem-fir, SYP, SPF or DougFir - Larch. I'd use 2x10's. If the snow load is heavier then that needs to be looked at again. A 6x6 sitting on top of the pier would probably work, but my tables don't have square beams (because inch for inch more height gives much more strength/stiffness than more width).
Cut a notch out of the 6x6 piers tops, being very careful not to overcut... best to cut almost to the inside corner and then finish with a chisel. The idea is to make a shelf for the two 2x's to sit on, on edge, and to make the top edges of the 2x's level with the pier tops. Drill through and secure with machine bolts, large washers and nuts. Do not overtighten, do not crush the wood fibers. Nylok nuts won't loosen.
Yes, I would set the piers as you described packing the earth back in around the piers very well.
If the lower edge of the girders are at least 12 inches above grade the girder does NOT need to be PT wood. If the lower edge of the floor joists are greater than 18 inches above grade they do NOT need to be PT wood. Same for the plywood underneath the floor joists, if higher than 18" from grade the plywood can be standard exterior grade... NO USB.
Floor joists; I'd choose 2x12 #2 at 16" centers, no center girder. Any cost savings in being able to use smaller joists is negated with extra piers, the extra girder and the extra labor, IMO.
Or do a small amount of cantilever with the floor joists and use 2x10 #2 on 16" centers. Ten to twelve inches one both sides would shorter the joist span enough to make 2x10's work well. Thos is only recommended if the upper floor is just a sleeping loft with no extra heavy items; no gun safes or water beds.
And remember to brace the piers very well before starting on the walls.
Below are two shots of some near excellent bracing. The only niggling improvement would be to square cut the upward directed acute angle. As it sits it sort of acts like a chisel; cut square it would not. (Not my work, but it's a design of a friend who's work I admire.) This is in Nicaragua. The tightly fitted members resist movement in compression; the fasteners only hold the pieces in place and have no shear forces applied to them.
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MtnDon
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# Posted: 31 Jan 2012 11:11pm
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Any reason for 2x4 walls? They are strong enough but a bit thin on insulation value if batts are used. Two inches of exterior foam would help that a lot though.
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cabinbiscuits
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# Posted: 31 Jan 2012 11:34pm
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MtnDon- Thanks a lot for your help. That is some really nice work that your friend did. Was that a mission trip?
What would be sufficient for bracing the piers 2x4, 2x6, something else?
Do they need to be notched into the piers like your friend did, or can they just be surface nailed, lagged, or bolted onto the back sides of the piers? I don't think I could make near as nice joints as your friend did.
I like the idea of the 2x10's for the girders. I don't know if they are readily available in 20' lengths or not. Is is permissible to use 16's and 4's with the splices over piers on opposite ends of the building?
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cabinbiscuits
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# Posted: 31 Jan 2012 11:45pm
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Quoting: MtnDon Any reason for 2x4 walls? They are strong enough but a bit thin on insulation value if batts are used. Two inches of exterior foam would help that a lot though.
The main reason for the 2x4's is for cost savings. I agree with you totally about the insulation, as a matter of fact, when I re-sided my house that has 2x4 walls I used 2" foam on the outside prior to putting the new siding on, in order to pick up the R-Value of the walls.
In this case I'm trying to keep the build as cheap as possible, so that in the event of fire, either by vandals, by forest fire, or by accident I guess, the loss would be less. The building will be totally off grid, and since it will be on top of a mountain, there is no way that a fire truck (even an off road brush truck) would ever be able to get up to it.
Since we'll be heating with a wood stove, I don't think that loss of insulation value will be as much of an issue as it would if we were going to keep it heated with some other source while we weren't there.
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MtnDon
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# Posted: 1 Feb 2012 12:39am
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Quoting: cabinbiscuits Was that a mission trip?
No, just a gringo living down there. :)
Those braces in the pictures are more like 3" thick. Much stiffer than 2x... is there a saw mille near by? You could have then rip 6x6 down
The whole idea of the notching is to eliminate the need for relying on mechanical fasteners for strength. Those do take some patience to make. Those in the photos were cut by local native workmen, to the design supplied to them. There's a drawing below that would be easier to implement. Smaller notches and the brace length is not as critical as the filler piece is cut to fit after the diagonals are set. The block would be a block of 2x placed with the grain lengthwise, inline with the direction of grain in the girder. The diagonals could be 4x6.
If good 20 ft 2x10's can not be located then splice shorter lengths. It may be better to build the girder from ten foot lengths. It is preferred to not have the splices all over the same pier. However it is also preferred to not have a short section spanning only two piers at the end. Dilemma. The reason against the short 2 pier span is because the second pier in from and end is almost always more heavily loaded than the end pier or the central pier(s). So it's desirable to have no splices over the second pier in from the end. You could help keep things together at the center pier where all the splices would be with 10 foot lengths, by bolting a steel plate over the splice.
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MtnDon
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# Posted: 1 Feb 2012 12:43am
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You can help out the wildfire situation by using cement fiber siding and trim. With a gambrel roof line you avoid the eve overhang that can be a place where fires can grab a toehold. We planned our place with wildfire threats in mind... the exterior is almost all cement fiber.
You'd be surprised where the USFS F450 engines can get to. I have seen them in action around here (regrettably all too close too)
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cabinbiscuits
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# Posted: 1 Feb 2012 11:03am
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I was thinking last night while lying in bed trying to go to sleep, if the 2x10 girders are notched and bolted into the tops of the piers, wouldn't the fit of those joints serve to counteract any lateral movements of the piers in the same way as the bracing, or is the bracing still required? Like I said the girders should be no more than 24" to the top above grade, so the piers will only be like 1 to 2 feet above the grade give or take dependent on the grade at any give spot.
I've read a bunch of your posts and greatly respect your knowledge and opinion, so I'm going to do whatever you say is the right way to do it, this was just something that occurred to me as I was thinking through the process.
All of the fire departments in the area of our camp land are small volunteer departments, and to be honest, I'm not even sure if they have an off-road brush truck. I guess it wouldn't hurt to check it out in advance. I thought that even with one they wouldn't be able to get past the gate that I'm putting in, but I guess in the event of a fire they would probably just cut the lock anyway. If they have locking axles on their trucks and the trucks were small enough they could make it to the top of the mountain on my camp road. Which would obviously be fine with me. I think at some point I will reach out to the local fire chief and show him my camp road if he's interested in case they should ever need to use it.
i looked at the cement siding for my house, but my budget is pretty tight for this cabin. My other thought is that if vandals were to set fire from the inside, then that wouldn't help, and would just be more money lost. I'm not sure why I'm so worried about someone burning the place down, I guess after reading some of the posts on here though it's certainly possible. I plan on securing the place pretty well to keep intruders out, so for some people that will just make them mad and it might make them want to just destroy it if they can't get in or have to work too hard to get in.
My plan as of right now is to use Smart Side panels for the outside.
I'm still not 100% firm on the 2x4's instead of 2x6's, I have to look at the cost difference, but the more I think about it I bet it's only a couple hundred dollars which might be worth the extra insulation it in the middle of the night when the fire is burning out. The other thing would be the jam extensions on the windows, but I don't think those would be that much either, or I could just make my own.
Thanks again.
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cabinbiscuits
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# Posted: 1 Feb 2012 12:40pm
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No need for girder splices I called around and found out that 84 Lumber has pressure treated 2x10's and 2x12's up to 24' long. Sweet....
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MtnDon
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# Posted: 1 Feb 2012 03:08pm
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The let in girder helps with making the pier/girder connection resist movement. But if you were to ask a structural engineer about a pier/girder foundation most of them will advise not to do it at all because it's difficult to make them truly as strong as a concrete block or pored concrete foundation, or a slab. I understand why many builders want to use them; I did myself. But they need bracing and most cabins are deficient. They also need bracing across the width, more so than down the length and that is more difficult to achieve. Being able to align joists with the piers helps as them bracing can extend from a pier up to a joist and then back down to a pier on the other side. Shear walls can also be built between the piers on both ends.
An engineer would also have a fit over piers extending 2 feet in the air and only 4 feet in the ground if there was no bracing or inadequate bracing. Engineers are paid to worry about that stuff as they or their insurance has to pay out when something fails.
Basically the piers should be as short as possible above ground and as deep as possible below ground. And they should be braced so nobody's depending on the ground to hold the pier in place laterally. You never know what's going to happen to the ground. And it is a devil of a job to change or repair later.
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